Other mistake: In the scene where Jamie is in the closet, looking at a photo of her mother (Jamie Lee Curtis). The photo looks exactly like the scene where Laurie is sitting waiting for Annie on the ledge in the original Halloween, which wasn't photographed at the time, it's clearly a still from the movie.
Halloween 4 (1988)
Ending / spoiler
Directed by: Dwight H. Little
Starring: Donald Pleasence, Danielle Harris, Ellie Cornell, George P. Wilbur
The police shoot Michael Myers in the torso with a shotgun. He is blown back and falls down a mine shaft. The cops throw a stick of dynamite down the shaft which causes it to cave in. Later that night, Jamie Lloyd "becomes" Michael Myers and kills her stepmother with a pair of scissors.
Poison Gom Jabbar
More mistakes in Halloween 4
Dr. Loomis: I need to speak with Sheriff Brackett.
Deputy Pierce: Well, then you'll have to travel about three-thousand miles south of here.
Dr. Loomis: What?
Deputy Pierce: Brackett retired in '81. He went to St. Petersburg.
Trivia: Series creator John Carpenter actually co-wrote an early draft of the script and even considered directing, but his ideas were dismissed by producers as being "too cerebral" for audiences, and he subsequently dropped out of the project. His script reportedly was more focused on the psychological effect Michael Myers had on the residents of Haddonfield (and possibly inspiring other murders), with Michael only appearing in visions, dreams and possibly as a "spirit."
Answer: Likely because keeping Michael in a maximum security facility was seen as a waste of time/resources given he had been in a coma for years.
TedStixon